In vitro and in vivo interactions of [3H]dimethylstilbestrol with the estrogen receptor

Biphasic actions of estrogen on colon cancer cell growth: possible mediation by high- and low-affinity estrogen binding sites

The present experiments were carried out to investigate the possible direct effects of estrogens (E) on the growth of colon cancer cells. Estradiol exhibited a concentration-dependent biphasic growth effect on a mouse colon cancer cell line (MC-26). Low concentrations of estradiol (10(-10) M: to 10(-8) M: ) had a growth-stimulatory effect, while higher concentrations (10(-7) M: to 10(-6) M: ) were growth-inhibitory. Estrogen receptor (ER) mRNA as well as specific, saturable binding of estradiol to ER (K(d)=0.3NM: , B(max)=0.72 fmol/μg DNA) was identified in these cells. In addition to the classical high affinity ER, lower affinity, higher capacity estrogen binding sites (K(d)=35MM: , B(max)=30 fmol/μg DNA) were also characterized in MC-26 cells. These two types of estrogen binding sites exhibited distinct binding specificities for E and antiestrogens. Treatment of MC-26 cells with an oligodeoxy-nucleotide antisense to the translation start codon of ER mRNA did not alter the grown-inhibitory effect of 10(-6) M: estradiol, demonstrating that the growth-inhibitory effect of high concentrations of E was not mediated by ER; we have previously shown that under the same conditions, ER antisense oligonucleotides do block the growth-stimulatory effects of 10(-9) M: E(2) in MC-26 cells. The data suggest that physiological concentrations of estradiol acting via the classical ER may have a proliferative effect on the growth of colon cancer cells. However, in situations where there are high luminal concentrations of estrogenic compounds, they may act on low affinity estrogen binding sites that mediate the growth-inhibitory effect.

Relative mitogenic activities of various estrogens and antiestrogens

The abilities of a variety of estrogens and antiestrogens to stimulate DNA synthesis in the prepuberal rat uterus were compared. One microgram of each compound was administered in vivo via a single intraperitoneal injection. DNA synthesis was assayed in vitro in isolated nuclei 24 h later. The relative mitogenicities of the steroidal estrogens were: 16 alpha-E2 less than 17 alpha-E2 = E3 = 16-EpiE3 less than 16 beta-E2 = 17 beta-E2. The potencies of several nonsteroidal estrogens were also tested. Indenestrol A was as potent at 17 beta-E2, whereas indanestrol and dimethylstilbestrol had weaker activities. The antiestrogens, nafoxidine and 4-hydroxytamoxifen, were both potent stimulators of DNA synthesis. The abilities of an estrogen to stimulate increases in uterine wet weight, DNA polymerase alpha activities, and DNA synthesis in uterine nuclei 24 h after injection were closely correlated. Because the magnitude of the stimulation of DNA synthesis was greatest, its measurement is the most sensitive of these assays of uterotrophic activity.

The effect of prostaglandin F2 alpha treatment on the endometrial oestradiol receptor system

The effects of prostaglandin F2 alpha treatment on oestradiol receptor levels were studied in human decidual tissues from the first trimester of pregnancy. Two types of nuclear oestradiol binding sites in decidual tissue were detected. Type I receptors bind the oestradiol with high affinity and low capacity (Kd = 1.4 nM, Bmax = 0.3 pmol/mgDNA, Hillcoeff = 1), while type II binding sites have lower affinity and higher capacity for the hormone (Kd = 20.4 nM, Bmax = 2.0 pmol/mgDNA, Hillcoeff = 3). The nuclear oestrogen binding capacities of decidual tissue were affected by prostaglandin F2 alpha in a time dependent manner. Within the first 10 hrs after prostaglandin F2 alpha application the concentration of low affinity, high capacity binding sites (type II) was enhanced in comparison with the control values. Later, 20-24 hrs after the treatment, both types of nuclear oestrogen binding sites were increased significantly. When prostaglandin F2 alpha treatment failed to induce abortion no change in the concentration of nuclear oestrogen binding sites was observed. Oestradiol receptor concentration in the cytoplasm apparently was not affected by prostaglandin F2 alpha treatment.

The agonistic and antagonistic actions of estriol

Estriol is a short acting estrogen, and as such, displays both agonistic and antagonistic properties, when it is injected in saline solution. These are predictable attributes of any short acting agonist if one considers the basic pharmacology of receptor ligand binding interactions. That is, short acting hormones or drugs occupy receptor sites for protracted time periods and, although they stimulate responses, these are of short duration. However, when estriol is administered in continuous fashion the estrogen receptor is occupied for long periods of time and full estrogenic responses are observed, and there is no antagonism of estradiol action. The purpose of this paper is to review the work that leads to the above conclusion and to discuss our more recent work on the relationship between estrogen receptor binding and the stimulation of type II estrogen binding sites.

High-affinity binding to the estrogen receptor of [3H]4-hydroxytamoxifen, an active antiestrogen metabolite

The tritiated 4-hydroxytamoxifen (OHT), an active metabolite of tamoxifen found in estrogen target nuclei, was prepared in vitro and its interaction with the cytosol estrogen receptor (R) of uterus and chick oviduct was specified. OHT bound to the 8S R with the same affinity and the same kinetic parameters as estradiol (E2). Its ability to protect the E2-binding sites and to stabilize the R--DNA interaction was also similar to that of E2. We conclude that the binding characteristics in vitro of ligands on the cytosol R do not presently allow us to discriminate between agonist and antagonist ligands.

High-affinity binding of the antiestrogen [3H]tamoxifen to the 8S estradiol receptor

The interaction of tamoxifen (ICI 46,474), a synthetic antiestrogen, with uterine cytosol proteins of immature calf and rat has been studied directly using the tritiated compound labeled with a high specific activity. The binding complexes were measured by the dextrancoated charcoal, protamine sulfate and hydroxyapatite assays. Scatchard plots revealed a single class of high-affinity (KD congruent to 1.7 nM) binding sites, with a binding capacity similar to that of estradiol. Competitive experiments showed the same binding specificity for estrogens and antiestrogens. Sucrose gradient analysis revealed an 8S binding protein which could be partially proteolysed by trypsin into a 4S binding protein. Kinetic studies showed that the association rate of tamoxifen was 5 times lower than that of estradiol and reacted according to a second order kinetics. The first-order kinetics of dissociation was considerably higher than that of estradiol, giving a half-dissociation time of 20--40 min at 0--2 degrees C. In some cases tamoxifen displayed two slopes of dissociation, but the proportion of the slow-dissociating complex was always inferior to that found with estradiol. In contrast to estradiol, the kinetic constants ratio (k-/k+) gave a calculated dissociation constant, similar to that determined in equilibrium conditions (KD), agreeing with a simple reactional scheme. We conclude that the antiestrogen tamoxifen binds directly to the 8S cytosol receptor for estrogens and not to another receptor for the antagonists. In contrast to estradiol, the antagonist is rapidly dissociated from the receptor sites and is unable to protect them against thermal inactivation. The affinity of tamoxifen for its receptor sites as determined directly is surprisingly high when compared to its affinity evaluated indirectly by competitive experiments. It is then suggested that the two ligands either bind on two different sites of the same protein or induce a different conformational change of the same binding site.